Odd question. Directions?

[Daemian]

sides? sideways? laterals? cross sounds good for me.

both red and blue are horizontal, but only the red are diagonal. I don't know what the blue ones are called.

I guess if you have your plane like floating in space without sense of direction, all lines are horizontal or vertical until you decide which is which and turn the plane to the angle you picked.
I know in chess, being a 2d plane, they pick whites' side as the base and they still use vertical, diagonal, horizontal to name the movements the pieces do.